The invention relates to the field of solar power collection. More particularly, the present inventor relates solar collectors, solar concentrators, and spectrum splitters for improved collection solar power collection efficiency.
Solar cells are photovoltaic devices that convert sun light into electricity. In order for photovoltaic devices to significantly contribute to the nationwide energy supply, both the cost per installed kilowatt of photovoltaic generating capacity and the price charged for a kilowatt-hour of electricity generated from such devices, must be reduced. Current research extends in two directions, including solar concentration and spectrum splitting. Efficiencies in solar power generation result from concentration and/or spectrum splitting.
Prior solar light concentrators have used either one axis or two axis standard Fresnel lenses, curved prismatic lenses, curved mirrors or other reflective and or refractive optical devices to concentrate solar energy for conversion to electricity using the photovoltaic cells. These photovoltaic cells are commonly referred to as solar cells. The prior optical concentrator devices enable a significant reduction in the solar cell area to be achieved for a given level of power output and thus greatly reduce the costs associated with the otherwise very expensive solar cells. At a specified operating temperature, higher sunlight to electricity conversion efficiencies are achieved by employing solar cells designed for operation at the higher concentration ratios.
Due to the finite size of the sun, practical limits of solar energy concentration are approximately one hundred times per axis of concentration with both reflecting and refractive lenses, such as a Fresnel lens. Concentrators designed for a single axis of concentration can achieve a one hundred suns concentration ratio whereas two axis concentrators can achieve up to a 10K suns concentration ratio. Practical solar cells that are designed for high concentration conversion operate in the range of 0.1K to 1K suns concentration ratio. In general, the higher the concentration ratio to be achieved, the more stringent are the optics accuracy and the pointing requirements with respect to the direction of received sun light.
A Fresnel lens is characterized as a sawtooth refractive optical lens. A Fresnel lens is disclosed in U.S. Pat. No. 4,069,812. The Fresnel lens has been applied to solar energy collection. The Fresnel lens may be a curved prismatic type lens that performs that function of spectrum splitting the received sunlight. When designed for spectrum splitting the practical concentration ratio is reduced from approximately one hundred suns to approximately twenty suns. This reduction in concentration ratio is because spectrum splitting the sunlight spreads the incoming sunlight into its component frequencies.
Although many concentrator devices are capable of achieving high concentration ratios, the ultimate efficiency and related economics are limited by the conversion performance of the associated solar cells. Solar cells are very efficient at converting light to electrical energy at a single bandgap optimized frequency. However, sunlight is composed of a wide spectrum of different frequencies. The overall efficiency of the solar cells diminishes rapidly at frequencies that are above or below the bandgap frequency of the selected solar cells. Many proposals have been made to increase solar array efficiency by using two or more solar cells with appropriately spaced bandgaps to span a greater portion of the incident solar spectrum. Each bandgap is selected to best match the input spectral portion and thus obtain maximum efficiency. Traditional design practices address this problem by developing solar cells that can be vertically stacked. Ideally, each cell in the stack is optimized for a specific frequency and associated solar cell bandgap. However, this bandgap stacking approach presents numerous design challenges that are primarily associated with solar cell substrate mismatches, frequency response and transmission characteristics of the cell materials, and electrical compatibility of the various cell characteristics. Designs directed to meet a number of challenging design factors associated with vertically stacked cells, are limited by the number of bandgap junctions that can be stacked using conventional semiconductor thin film processes. Ideally, each bandgap is optimized for a different frequency response that accumulates into a spectral range. Hence, the spectral range is limited when using a limited number of bandgap junctions, for example, between two to four bandgap junctions that can be achieved in a vertical stack using conventional semiconductor fabrication processes. In order to satisfy substrate requirements, additional compromises are made in the design and frequency characteristics of the stack. Furthermore, the bandgap stacking design approach disadvantageously have complex multilayer and multistep manufacturing processes that result in expensive solar cells with significantly lower than theoretically achievable efficiencies. Due to the complexity and manufacturing difficulty of multijunction vertically stacked solar cells, current devices operate with only two or three bandgaps. Technology limits may soon reach a viable four bandgap solar cell configuration. Ultimately, the vertically stacked approach is limited in the number of bandgaps and associated conversion efficiencies that can be achieved. Due to the complex multilayer multistep manufacturing processes involved, the vertical junction design also increases the production costs.
Solar cell designers recognized that when the solar cells are separated spatially, that is horizontally rather than vertically overlaid, each cell can be separately designed and manufactured on respective unique and optimized substrates. This respective substrate processing technique would eliminate the complex manufacturing processes and inefficiencies associated with vertically stacked solar cells. Each solar cell could be individually optimized for a specific bandgap optimized for a small portion of the solar spectrum without concern as to substrate mismatch and process design impediments of juxtaposed solar cells within an array of frequency sensitive solar cells. In addition, the horizontal spatial arrangement would allow for the integration of much larger number of different bandgaps, for example, six to ten. With proper design optimization, 60% to 70% of sunlight to electricity conversion efficiency is achievable. The horizontal spatial arrangement also allows design of employing spatially separated cells of different bandgaps to share a common substrate by selection of different cell materials and optimizing the design for each desired bandgap of response. For the horizontal spatial arrangement approach to suitably function, an additional step occurs in the optical process prior to reaching the solar cells. The solar spectrum must be split into respective frequencies and then optically redirected to each respective recipient solar cell. The optical redirection can be accomplished with a prism based optics system. The prism based optic system can be manufactured in the form of a Fresnel lens. In such a prism optical system, the prism device provides spectrum splitting with limited concentration in a single axis. The employment of a prism to split the solar spectrum decreases the ultimate concentration ratio that could otherwise be achieved. As a consequence, the limited achievable concentration of the prism optics with the spectrum splitting approach offsets the advantages over higher concentration.
The combination of both high concentration and spectrum splitting would allow for the development of a low cost solar array capable of leveraging the benefits of both features for minimizing solar cell area with high conversion efficiency. Prior collector systems with either high concentration ratio or high efficiency spectrum splitting conversion have been used but not effectively combined together for improved collector efficiency of solar energy. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide improved efficiency of solar power collection and conversion.
Another object of the invention is to provide collected solar power using solar light concentration and solar light spectrum splitting.
Still another object of the invention is to provide solar power conversion using reflective solar light concentration with spectrum splitting.
Yet another object of the invention is to provide solar power conversion using refractive solar light concentration with spectrum splitting.
A further object of the invention is to provide a solar concentrator and energy conversion device that provides both the advantages of very high concentration ratios and conversion efficiency using an array of horizontally disposed bandgap optimized solar cells.
The present invention is directed to an optical arrangement in which a first axis of solar light concentration employs traditional reflective or refractive devices, and in which a second axis provides secondary concentration and prism spectrum splitting. The first axis of solar light concentration employs traditional reflective or refractive devices such as traditional Fresnel lenses, curved prismatic Fresnel type lens, or mirrors. The second axis of spectrum splitting can be perpendicular to the first axis of concentration. In the preferred form, a two lens system or mirror lens system has one axis lens providing a high degree of concentration and the perpendicular axis providing a generally lower level of concentration as well as also providing the spectrum splitting function. Although both the axis of concentration and the axis of spectrum splitting could be achieved in a single combined Fresnel and prism lens, the two lens system provides an economical optical design.
The optical system enables a high concentration ratio and spectrum splitting concentrator that can be manufactured at low cost in various optical configurations. A combination of optics means concentrate sunlight on photovoltaic cells providing efficient energy conversion to electricity. The collector system uses concentrating optics, spectrum splitters, and bandgap optimized solar cells. Preferably, solar energy is concentrated in the first axis and split into frequency components along the orthogonal second axis and then directed to frequency sensitive photovoltaic cells horizontally aligned in subarrays and designed to operate at a high concentration ratio. Incident solar energy is firstly concentrated and then secondly spectrum split and then efficiently converted by the frequency sensitive horizontally aligned photovoltaic solar cells integrated into a solar panel. The optical devices concentrate the solar energy and then split the solar spectrum for optimum illumination of the horizontally aligned photovoltaic solar cells aligned in a plurality of horizontally aligned subarrays. The solar cells are horizontally disposed in the subarrays for receiving light at respective frequencies near optimal for each of the different bandgap optimized solar cells in the subarrays. Each horizontal subarray converts light of a respective frequency into a respective voltage. A plurality of horizontally aligned subarrays can be buttressed together forming a matrix solar panel with the subarrays having rows of identical bandgap optimized solar cells. Each horizontal row of solar cells is optimized for a desired frequency response and output voltage. The size of the cells in each row may have an equal width and height and matched to the illumination frequency for providing identical electrical characteristics across the row. The spectrum of the light is distributed vertically across the rows of cells. The solar concentrator illuminates the solar panel having a plurality of single junction planar solar cells. The system preferably uses horizontally disposed solar cells manufactured within conventional manufacture limits using conventional semiconductor materials. The resulting photovoltaic panel is capable of converting highly concentrated spectrum split sunlight with high conversion efficiency and integrated in a single solar collector device.
The solar concentrator preferably uses a matrix solar array panel using a combination of the horizontally aligned subarrays of photovoltaic solar cells in conjunction with the spectrum splitting optics. The horizontally aligned photovoltaic subarrays of cells are optimized for specific bandgaps to achieve very high conversion efficiencies. In the exemplar form, each subarray uses single junction cells responsive to the same frequency. Each subarray of cells is optimized for a single operating bandgap for a respective frequency of an associated solar light wavelength. The spectrum splitting concentrator can achieve conversion efficiencies between 45% and 60%. The preferred form offers a high concentration ratio, for example between 100X to 500X, to enable a significant reduction in the acreage of expensive solar cells. The use of highly concentrated sunlight on optimized single bandgap cells enables a reduction in the manufacturing cost per unit area of the solar cell panel. The combined effect of high efficiency energy conversion and high concentration ratio reduces the required area of the costly photovoltaic cells. The system offers the cost advantages of high concentration ratio and of efficient multispectral energy conversion using bandgap optimized photovoltaic conversion. The system is capable of being manufactured at a lower cost per kW of installed capacity.
A wide variety of concentration optical techniques may be utilized, including one or two axis standard Fresnel lenses, curved mirrors, or other reflective and or refractive devices. The first and second lenses can employ conventional reflective or refractive optics. The lenses may be flat or curved. The first lens performs a high degree of concentration in one axis. The second lens may be a prismatic Fresnel lens capable of spectrum splitting. The spectrum splitting lens may provide a small degree of concentration to increase the overall effective concentration.
The collector system is capable of achieving a high concentration ratio along the first axis while incident solar energy is spectrum split and further concentrated in the second single axis. The combined concentration of both axes provides a high concentration ratio, for example, 500 suns. The first axis of concentration achieves a high concentration ratio whereas the second axis of concentration increases the effective concentration ratio, for example, by thirty times with spectrum splitting for improved conversion efficiency. Economic advantages of high efficiency reduce required photovoltaic acreage to offset the high cost of multifunction photovoltaic solar cells. The highly concentrated sunlight with minimized photovoltaic cell acreage achieves high efficiency to maximize power output while also reducing manufacturing costs. The high efficiency energy conversion enables photovoltaic energy to be generated at costs approaching competitiveness with fossil fuel burning energy sources. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.